Lithographic apparatus for imaging a front side or a back side of a substrate, method of substrate identification, device manufacturing method, substrate, and computer program

ABSTRACT

The invention is directed to enabling substrate identification by comparing the measured distance between two features on an unidentified substrate with one or more stored distances. The one or more stored distances are the distances intended during the design of one or more substrates. The unidentified substrate is identified by a stored distance that corresponds to the measured distance. The two features are selected from a plurality of features that may be placed on a back side or a front side of a substrate. An optical system is provided for reading the features from the back side or a front side of the substrate.

This application claims priority from and is a continuation-in-part ofU.S. patent application Ser. No. 10/790,252 filed on Mar. 2, 2004 whichis now abandoned and also claims priority from and is acontinuation-in-part of U.S. patent application Ser. No. 10/954,654filed on Oct. 1, 2004, which is now U.S. Pat. No. 7,177,009 both ofwhich are herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to substrate measurement that is performed on afront side or a back side of a substrate.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning structure, which is alternativelyreferred to as a mask or a reticle, may be used to generate a circuitpattern corresponding to an individual layer of the IC, and this patterncan be imaged onto a target portion (e.g. comprising part of, one orseveral dies) on a substrate (e.g. a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include so-called steppers, inwhich each target portion is irradiated by exposing an entire patternonto the target portion in one go, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through theprojection beam in a given direction (the “scanning”-direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

During production of integrated circuits, a substrate is typically fedinto a lithographic apparatus several times in order to be able toproduce a circuit which consists of several layers on top of each other.As many as 30 layers can be used. The lithographic apparatus used togenerate the circuit pattern in the first layer is typically not thesame as the lithographic apparatus used to generate the circuit patternin the final layer. This is because the features of the circuit patternin the final layer are typically much larger than the features in thefirst layer, so that a less accurate and therefore less expensivelithographic apparatus can be used to apply the desired circuit patternto the final layer.

Conventionally substrates are provided with alignment marks whosepositions relative to the target portions are known. During alignment,an alignment sensor measures the positions of the alignment marks. Inthis way, the positions of the target portions may be determined. Thealignment sensor views a small area on the substrate at a given time,the small area being considered the footprint of the alignment sensor.Often, when alignment is begun, the alignment mark does not coincidewith the area viewed by the alignment sensor. To solve this problem, thesubstrate is scanned underneath the alignment sensor, over a distancesufficiently large to make certain that the alignment mark passesthrough the area viewed by the alignment sensor. The position of thealignment mark is measured as it passes through the area viewed by thealignment sensor.

A possible disadvantage of this alignment method is that scanning of thesubstrate is time consuming, and thus effects the number of substratesthat can be processed by the lithographic apparatus per hour.

Instead of using alignment marks, U.S. Pat. No. 3,898,617 describes analignment system that measures positions of circuit features so thatalignment is performed using circuit features. The alignment sensorviews a small area of a target portion on the substrate. The sensorrecords an image of the circuit features located in the area of thetarget portion. The image is compared to a library which includes imagesof circuit features and their associated locations. When a match isfound between the measured image and a library image, the associatedlocation retrieved from the library gives the location of the substrate.

A possible disadvantage of this method is that a time consuming scan ofthe substrate underneath the alignment sensor may be needed until arecorded image is found which matches an image in the library.

Once alignment is achieved, several lithographic apparatuses may beinvolved to simultaneously apply the desired circuit pattern onto thefirst layer or any other layer of the substrate. Even though thecalibrations of the machines are performed as accurately as possible,each apparatus may introduce its own errors. These errors may adverselyaffect the image applied to the substrate or the position of the imageon the substrate. In the event a lithographic apparatus is calibratedbetween two sets of substrates (commonly referred to as lots), the errormay also differ for the two sets.

When two lithographic apparatuses are used simultaneously, this impliesthat there may be several patterning structures available containing thepattern to be applied to a given layer. The several patterningstructures may also differ due to production tolerances. Thesedifferences may lead to differences in the images applied to thesubstrate or to differences in positions on the substrates where theimages are applied.

Commonly the substrates are marked with a code that is scratched intothe substrate. The substrates can be identified using these codes. Therelation between the identities of the substrates and the lithographicapparatuses or the patterning structures used to project images onto thesubstrate is stored. The combination of the stored relations and theidentity of the substrates can be used to correct for the differences,based upon knowledge of the previously used lithographic apparatus orpatterning structure.

However, a special sensor is required in the lithographic apparatus toread the code in order to identify the substrate or determine theprocess steps applied to the substrate. This has effects on the costs ofthe lithographic apparatus and on the throughput since time is needed toread the code. Given the relatively high cost of a lithographicapparatus compared to the other machines used during the totalproduction of integrated circuits, there are a limited number oflithographic apparatuses available to the system, so the throughput ofthe lithographic apparatus is typically the bottleneck in the productionprocess.

In addition, placing codes on substrates has been limited because thespace used to print the codes takes away from the valuable space thatmay be used to manufacture integrated circuits. In order to increaseefficiency and reduce cost, it is desirable to manufacture additionalintegrated circuits on the substrate without increasing the size of thesubstrate. Producing integrated circuit may be made less expensive perintegrated circuit or faster per integrated circuit when more integratedcircuits are placed on one substrate. Therefore, the production costscan be decreased and the throughput can be increased by freeing space onthe substrate for extra integrated circuits and refraining fromreserving space of codes.

Known methods for increasing substrate target areas have includedplacing alignment marks on a back side, or second side, of thesubstrate, which is opposite to a front side, or first side, of thesubstrate. Typically, the front side includes the integrated circuit. Alithographic apparatus that includes an optical system which is capableof directing alignment radiation to the back side of the substrate isdisclosed in U.S. Pat. No. 6,768,539, herein incorporated by referencein its entirety. The image of the alignment mark may be provided at aplane of the first side of the substrate. This enables a commonalignment system to be used for alignment of marks on both sides of thesubstrate. The alignment system may be capable of performing alignmentusing features on the front and back of a substrate.

SUMMARY OF THE INVENTION

In a lithographic production process, markers or features may beconfigured to communicate various properties associated with thecorresponding substrates. In one embodiment of the invention, thefeatures may include alignment markers that provide alignmentinformation or other information associated with the substrate. Thesystem may be capable of identifying the substrate based on propertiesof the features and/or identifying the type of process that is to beapplied to the substrate based on properties of the features. In anotherembodiment, the system may derive information from the features toautomatically adjust alignment values of the system on a per substratebasis, a per lot basis or other quantity of substrates.

In a further embodiment, features may be arranged on a substrate toconvey information based on spatial coordinates of a plurality offeatures. For example, a plurality of features may be used to generate acode using position offsets. In another embodiment of the invention, thefeatures may be oriented relative to primary alignment markers in orderto facilitate locating the features. In yet another embodiment of theinvention, the features may be positioned on a first side or a secondside of a substrate.

A lithographic apparatus according to one embodiment of the inventionincludes at least one sensor arranged to measure positions of first andsecond features located on a first side or a second side of a substrate,and an identification unit arranged to compare a measured relativeposition of the first and second features based on the measuredpositions with at least one of a plurality of stored relative positionsof first and second features. Each of the plurality of stored relativepositions of first and second features is associated with informationcharacterising at least one substrate. The identification unit is alsoarranged to indicate a correspondence between the measured relativeposition of the first and second features and one of the plurality ofstored relative positions of first and second features.

A method according to another embodiment of the invention includesmeasuring positions of first and second features that are located on afirst side or a second side of the substrate. The method also includescomparing a measured relative position between the first and secondfeatures on the substrate, based on the measured positions, with atleast one of a plurality of stored relative positions of first andsecond features. Each of the plurality of stored relative positions offirst and second features is associated with information characterizingat least one substrate. The method also includes indicating acorrespondence between the measured relative position of the first andsecond features on the substrate and one of the plurality of storedrelative positions of first and second features.

A method of labelling a substrate according to another embodiment of theinvention includes providing a first side or a second side of thesubstrate with a first feature; providing the corresponding first sideor second side of the substrate with a second feature; and recording acorrespondence between a relative position of the first and secondfeatures and information characterizing the substrate. The informationmay distinguish the substrate from other substrates in a group and/orindicate membership of the substrate in a group.

A method of labelling a substrate according to another embodiment of theinvention includes providing a first side or a second side of thesubstrate with a first feature, and providing the corresponding firstside or second side of the substrate with a second feature at a positionrelative to the first feature so that the relative positions of thefirst and second features provide characterising information regardingthe substrate.

A lithographic apparatus according to another embodiment of theinvention includes one or more sensors arranged to measure the relativepositions of first and second features located on a first side or asecond side of a substrate, and an identification unit arranged tocompare the measured relative positions of the first and second featureson the substrate with one or more stored relative positions of the firstand second features. The one or more stored relative positions of thefirst and second features are each associated with informationcharacterising one or more substrates. The identification unit isarranged to determine if the measured relative positions of the firstand second features on the substrate correspond with one of the one ormore stored relative positions of the first and second features.

A device manufacturing method according to another embodiment of theinvention includes manufacturing a number of the devices on a set ofsubstrates. Each substrate is provided with a marker that is located ona first side or a second side of the substrate to provide informationregarding the position of the substrate. Each substrate is provided witha feature on a position relative to the marker such that the relativeposition is indicative of a setting of a process step of the substrate.

According to another embodiment, a method is provided for determining,in a lithographic apparatus having a sensor, a position of an objectprovided with a plurality of features having unique positions relativeto one another, including providing reference information indicating,relative to a position of one of the plurality of features, thepositions of the rest of the plurality of features; using the sensor tomeasure positions of each of a subset of the plurality of features, themeasured positions including a reference position in a coordinatesystem; identifying a feature in the subset, based upon the measuredposition of the feature relative to the other measured features; anddetermining a position of the object, based on the identity of theidentified feature, the reference information, and the measuredreference position in the coordinate system, wherein the plurality offeatures may be located on a first side or a second side of a substrate.

A lithographic apparatus according to another embodiment includes anobject table that is configured to support an object, the object beingprovided with a plurality of features having unique positions relativeto one another; a position sensor arranged to detect each of a subset ofthe plurality of features; a memory unit configured to store referenceinformation indicating, relative to a position of a reference feature ofthe plurality of features, the positions of the rest of the plurality offeatures; and a processing device, connected to the position sensor andto the memory unit, arranged to identify a feature in the subset basedupon a detected position of the feature relative to the other detectedfeatures, and arranged to determine a position of the object relative tothe sensor based on the reference information and a measured position ofthe reference feature in a coordinate system, wherein the plurality offeatures may be located on a first side or a second side of a substrate.

According to a further embodiment, a method is provided for determininga position of an object provided with a plurality of features, each ofthe plurality of features having a unique position in a two-dimensionalplane relative to any other two of the plurality of features, the methodincludes using a sensor of a lithographic apparatus to measure positionsof each of a subset of the plurality of features; identifying a featurein the subset, based upon the measured position of the feature relativeto the other measured features; and determining a position of theobject, based on (A) the identity of the identified feature, (B)reference information indicating, relative to a position of a referencefeature of the plurality of features, the positions of the rest of theplurality of features, and (C) a position of the reference feature in acoordinate system, wherein the plurality of features may be located on afirst side or a second side of a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 is a schematic cross section illustrating the substrate tableincorporating two branches of an optical system for double sidealignment according to an embodiment of the invention;

FIG. 3 is a plan view of a wafer showing the position and orientation ofthe double side alignment optics according to an embodiment of theinvention;

FIG. 4 is plan view showing an alternative position and orientation ofthe double side alignment optics according to the invention;

FIG. 5 is a cross section of a portion of a substrate table havingintegral optical components according to an embodiment of the invention;

FIG. 6 depicts a patterning structure containing a first circuitpattern, a reference mark and an alignment mark, which may be used toimplement an embodiment of the invention;

FIG. 7 shows a substrate having a substrate alignment mark, a firstcircuit pattern and a substrate reference mark on substrate W1 accordingto an embodiment of the invention;

FIG. 8 shows a substrate having a substrate alignment mark, a firstcircuit pattern and a substrate reference mark on substrate W2 accordingto an embodiment of the invention;

FIG. 9 shows an unidentified substrate having a substrate alignmentmark, a first circuit pattern and a substrate reference mark onsubstrate W2 according to an embodiment of the invention;

FIG. 10 shows the positions of a substrate alignment mark and asubstrate reference mark as defined, as realised in practice and asmeasured;

FIG. 11 depicts the lithographic apparatus of FIG. 1 in another crosssection;

FIG. 12 depicts an alignment region containing several features;

FIG. 13 depicts an image of a part of an alignment region containingseveral features;

FIG. 14 depicts an alignment region containing several features;

FIG. 15 depicts an image of a part of an alignment region containingseveral features;

FIG. 16 depicts two alignment regions each on a different object;

FIG. 17 depicts the image of a part of an alignment region containingseveral features.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention include methods and apparatusconfigured to provide substrate identification which solve one or moreproblems as described above.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCD's), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “patterning structure” used herein should be broadlyinterpreted as referring to structure that can be used to impart aprojection beam with a pattern in its cross-section such as to create apattern in a target portion of the substrate. It should be noted thatthe pattern imparted to the projection beam may not exactly correspondto the desired pattern in the target portion of the substrate.Generally, the pattern imparted to the projection beam will correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

Patterning structure may be transmissive or reflective. Examples ofpatterning structure include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The support structure supports, i.e. bares the weight of, the patterningstructure. It holds the patterning structure in a way depending on theorientation of the patterning structure, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning structure is held in a vacuum environment. The support can beusing mechanical clamping, vacuum, or other clamping techniques, forexample electrostatic clamping under vacuum conditions. The supportstructure may be a frame or a table, for example, which may be fixed ormovable as required and which may ensure that the patterning structureis at a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning structure”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art e.g. for effectively increasing the numericalaperture of projection systems.

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL for providing a projection beamPB of radiation (e.g. UV radiation or EUV radiation).

a first support structure (e.g. a mask table) MT for supportingpatterning structure (e.g. a mask) MA and connected to first positioningstructure PM for accurately positioning the patterning structure withrespect to item PL;

a substrate table (e.g. a wafer table) WT for holding a substrate (e.g.a resist-coated wafer) W and connected to second positioning structurePW for accurately positioning the substrate with respect to item PL; and

a projection system (e.g. a refractive projection lens) PL for imaging apattern imparted to the projection beam PB by patterning structure MAonto a target portion C (e.g. comprising one or more dies) of thesubstrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be an integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting structure AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the lens PL, which focuses the beam onto a target portionC of the substrate W. With the aid of the second positioning structurePW and position sensor IF (e.g. an interferometric device), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning structure PM and another position sensor (which is notexplicitly depicted in FIG. 1) can be used to accurately position themask MA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realised with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning structure PM and PW.However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C in one go (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning structure, and the substrate table WTis moved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning structureis updated as required after each movement of the substrate table WT orin between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilisesprogrammable patterning structure, such as a programmable mirror arrayof a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows a wafer W on a wafer table WT. Wafer marks WM3 and WM4 areprovided on a first side (“front side”) of the wafer W and light can bereflected from these marks, as indicated by the arrows above WM3 andWM4, and used for alignment with marks on a mask in conjunction with analignment system (not shown) which will be described later. Furtherwafer marks WM1 and WM2 are provided on the second side (“back side”) ofthe wafer W. An optical system is built into the wafer table WT forproviding optical access to the wafer marks WM1, WM2 on the back side ofthe wafer W. The optical system comprises a pair of arms 10A, 10B. Eacharm consists of two mirrors, 12, 14 and two lenses 16, 18. The mirrors12, 14 in each arm are inclined such that the sum of the angles thatthey make with the horizontal is 90 degrees. In this way, a beam oflight impinging vertically on one of the mirrors will remain verticalwhen reflected off the other mirror.

In use, light is directed from above the wafer table WT onto mirror 12,through lenses 16 and 18, onto mirror 14 and then onto the respectivewafer mark WM1, WM2. Light is reflected off portions of the wafer markand returns along the arm of the optical system via mirror 14, lenses 18and 16 and mirror 12. The mirrors 12, 14 and lenses 16, 18 are arrangedsuch that an image 20A, 20B of the wafer mark WM1, WM2 is formed at theplane of the front (top) surface of the wafer W, corresponding to thevertical position of any wafer marks WM3, WM4 provided on the front sideof the wafer W. The order of the lenses 16, 18 and the mirrors 12, 14can, of course be different, as appropriate to the optical system. Forexample, lens 18 could be between the mirror 14 and the wafer W (seeillustrations of later embodiments).

An image 20A, 20B of a wafer mark WM1, WM2 acts as a virtual wafer markand can be used for alignment by the pre-existing alignment system (notshown) in exactly the same way as a real wafer mark provided on thefront (top) side of the wafer W.

As shown in FIG. 2, the arms of the optical system 10A, 10B produceimages 20A, 20B which are displaced to the side of the wafer W so thatthey can be viewed by an alignment system above the wafer W. Twopossible orientations of the arms of the optical system 10A, 10B areshown in FIGS. 3 and 4, which are plan views of the wafer W, which liesin the XY plane. The wafer table WT is omitted from FIGS. 3 and 4 forclarity. In FIG. 3, the arms of the optical system 10A, 10B are alignedalong the X axis. In FIG. 4, the arms of the optical system 10A, 10B areparallel to the Y axis. In both cases, the wafer marks WM1, WM2 lie onthe X axis. The wafer marks WM1, WM2 are on the underside of the waferW, so are reversed from the point of view of the top side of the waferW. However, the arrangement of the mirrors of the arms of the opticalsystem mean that the images 20A, 20B of the wafer marks WM1, WM2 arerestored to be the correct way round again, not inverted, so that theimages appear exactly the same as if they were on the top side of thewafer W. The optical system is also arranged so that the ratio of thesize of a wafer mark WM1, WM2 to its image 20A, 20B is 1:1 i.e. there isno magnification or reduction. Consequently, the images 20A, 20B can beused exactly as if they were real wafer marks on the front side of thewafer W. A common alignment pattern or key provided on a mask can beused to perform alignment with both real and virtual wafer marks.

In the current example, wafer marks are provided on both the front andback sides of the wafer W at corresponding positions, as shown in FIG.2. In FIGS. 3 and 4, only the wafer marks on the back side of the waferW are shown, for clarity. According to this arrangement, when the waferW is flipped over, by rotation about either of the X or Y axes, a wafermark which was on the top side of the wafer W is now on the underside,but at a position such that it can be imaged by an arm of the opticalsystem 10A, 10B.

It will be noted that, because of the mirror arrangement, displacementof the wafer in one direction parallel to an arm 10A, 10B of the opticalsystem will displace the corresponding image 20A, 20B of a wafer markWM1, WM2 on the under side of the wafer in the opposite direction. Forexample, in FIG. 3, if the wafer W were displaced to the right, theimages 20A, 20B would be displaced to the left. Software controlling thealignment system takes this into account when determining the positionof the wafer marks WM1, WM2 and when adjusting the relative positions ofthe wafer W and a mask when performing alignment. If the two arms of theoptical system 10A, 10B are symmetric then the separation between theimages 20A and 20B will in fact stay constant when the wafer isdisplaced.

At least two wafer marks are provided per side of the wafer W. A singlemark can give information about the relative positioning of an image ofa specific point on a mask to a specific point on the wafer. However, toensure the correct orientational alignment and magnification, at leasttwo marks are used.

FIG. 5 shows a portion of the wafer table WT in cross section. Accordingto this embodiment of the invention, the optical system 10A, 10B forimaging the wafer marks on the back side of a wafer is built into thewafer table in a particular fashion. As shown in FIG. 5, the mirrors 12,14 of an arm of the optical system are not provided as discretecomponents, but are integral with the wafer table WT. Appropriate facesare machined into the wafer table WT, which may then be provided with acoating to improve reflectivity, thus forming the mirrors 12, 14. Theoptical system is made from the same material as the wafer table, such aZerodur™, which has a very low coefficient of thermal expansion andtherefore ensures that high alignment accuracy can be maintained.

Referring back to FIG. 1, the lithographic apparatus is represented witha rectangular co-ordinate system. In the co-ordinate system, thez-direction is defined as being parallel to the optical axis of theprojection system PL. The x and y co-ordinates are perpendicular to theoptical axis of the projection system. In this document for a finitearea (for example a target portion C), the side of the area with thelowest x co-ordinate is taken as the x co-ordinate of that area,although any other convention may be used, and different conventions maybe used for different areas if desired.

Alignment may be performed by measuring the position of substratealignment marks P1,P2 with an off-axis mark sensor MS. If themeasurement by the sensor is performed off-axis, the mark does not haveto be near the optical axis of the projection system PL. If themeasurement is performed off-axis, it may be desirable or necessary toknow or determine the relation between the position of the off-axismeasurement and the optical axis of the projection system PL. Positionsensor IF (e.g. a system including at least one interferometer or otheroptical or capacitive sensor) can be used to relate the position of theoff-axis measurement and the optical axis of the projection system PL.

Mark sensor MS is connected to an identification unit IU, which is alsoconnected to a first memory MEM1 and a second memory MEM2. The purposeof the first memory MEM1 is to store the defined positions of differentareas on the substrates. Defined positions in this document mean thepositions as engineered, i.e. as intended during design of alithographic step. In practice, the positions can deviate from thedefined positions. Likewise, in this document defined relative positionsare the engineered relative positions. In practice, the relativepositions can deviate from the defined relative positions, i.e. theintended relative positions. The purpose of the second memory MEM2 is tostore the relative positions of the different areas on the substrates.

The substrate alignment marks P1, P2 are areas formed on the substratewith a fixed position. These substrate alignment marks P1,P2 providereference positions for further process operations. By using the samereference positions in a first patterning operation as in a secondpatterning operation, the patterns in both operations can be positioneddirectly on top of each other.

In an embodiment of the invention illustrated in FIG. 6, the patterningstructure MA contains an alignment mark pattern M1@MA1, a first circuitpattern CP1@MA1 and a reference mark pattern M2@MA1, each covering aseparate area. The term ‘first circuit pattern’ is intended to mean thecircuit pattern of the first layer of the substrate.

In this document the names of the areas will contain an indication ofthe patterning structure or substrate on which they are formed. Thenames of the areas on patterning structure MA1 contain @MA1, the namesof the areas on patterning structure MA2 contain @MA2. The names of theareas on substrate W1 contain @W1 and the names of the areas onsubstrate W2 contain @W2. The x co-ordinate of an alignment mark patternM1@MA1 is xM1@MA1, the x co-ordinate of first circuit pattern CP1@MA1 isxCP1@MA1 and the x co-ordinate of reference mark pattern M2@MA1 isxM2@MA1.

The relative x co-ordinates of all three areas M1@MA1,CP1@MA1,M2@MA1 areknown. The names of the relative positions contain the same indicationof the patterning structure or substrate on which they are formed (forinstance @MA1). Furthermore the names are built up as dxAtoB@C wherein xindicates a direction in the co-ordinate system, A and B indicate whichareas are involved and C indicates on which patterning structure orsubstrate they are formed. Here the indication of the patterningstructure or substrate is left out of the name of the areas A and B.

As an example, the relative x co-ordinate between alignment mark patternM1@MA1 and reference mark pattern M2@MA1 is dxM1toM2@MA1.

In use, the lithographic apparatus may illuminate alignment mark patternM1@MA1, first circuit pattern CP1@MA1 and/or reference mark patternM2@MA1 separately, so that the images are projected onto the substrateseparately. This allows the position of each pattern as projected ontothe substrate to be selected independently.

As illustrated in FIG. 7, images of all three areas are projected onto asubstrate W by illuminating the patterning structure MA1. In practice,first circuit pattern CP1@MA1 is projected onto substrate W1 repeatedlyso that each target portion C is illuminated once. FIG. 7 only shows onetarget portion C for simplicity reasons. The images are projected ontodefined positions of substrate W1 being xP1@W1, xCP1@W1 and xP2@W1respectively (FIG. 7), which are stored in the first memory unit MEM(FIG. 1).

During the projection of the images onto substrate W1, there is aradiation-sensitive material on top of the substrate W1. Theradiation-sensitive material changes locally due to the energy in theprojected images.

After illumination, the substrate W1 will be processed. Duringprocessing local differences in the radiation-sensitive material per xco-ordinate are converted to presence or absence of semiconductingmaterial at the same x co-ordinate. Some or all of the processingprocedure may be performed outside the lithographic projectionapparatus. Following processing, substrate W1 contains a substratealignment mark P1@W1, a first circuit pattern CP1@W1 on substrate W1 anda substrate reference mark P2@W1.

The images of alignment mark P1@MA1, first circuit pattern CP1@MA1 andreference mark P2@MA1 are projected on a second substrate W2 using thesame lithographic apparatus. For the second substrate W2 the defined xco-ordinate of the substrate alignment mark is xP1@W2 (FIG. 8), which isalso stored in memory unit MEM1. This value differs from xP1@W1. Thedefined x co-ordinate xCP1@W2 for the first circuit pattern on substrateW2 is equal to that on substrate W1. The defined x co-ordinate xP2@W2 ofreference mark P2@W2 is also equal to that on substrate W1.

Substrate W2 is also processed. The substrate W2 now contains asubstrate alignment mark P1@W2, a first circuit pattern CP1 @W2 onsubstrate W2 and a substrate reference mark P2@W2.

For substrate W1, the distance dxP1toP2@W1 along the x-axis between theposition of the substrate reference mark P2@W1 and the substratealignment mark P1@W1 is calculated by the identification unit IU(FIG. 1) usingdxP1toP2@W1=xP1@W1−xP2@W1and stored as a first memory entry in the second memory unit MEM2(FIG. 1) which identifies substrate W1.

For substrate W2, the distance dxP1toP2@W2 along the x-axis between theposition of the substrate reference mark P2@W2 and the substratealignment mark P1 @W2 is calculated by the identification unit IU usingdxP1toP2@W2=xP1@W2−xP2@W2and stored in a second memory entry in the second memory unit MEM2,which identifies substrate W2.

The clear difference between the distances dxP1toP2@W1 and dxP1toP2@W2enables identification of substrate W1 and substrate W2.

An unidentified substrate WU (FIG. 9) is brought into the lithographicprojection apparatus to form a second circuit pattern CP2@WU on top ofthe first circuit pattern CP1@WU on substrate WU. In order to form thesecond circuit pattern CP2@WU exactly on top of the first circuitpattern CP1@WU, the position of the first circuit pattern CP1@WU onsubstrate WU must be determined. The substrate WU on substrate table WTis positioned in the measuring field of mark sensor MS (FIG. 1). Marksensor MS measures the position xP1@WU of substrate alignment mark P1@WUand position xP2@WU of substrate reference P2@WU.

The identification unit IU calculates the distance dxP1toP2@WU along thex-axis of the measured position xP1@WU of substrate alignment mark P1 WUand position xP2@WU of substrate alignment mark P2@WU.

The distance dxP1toP2@WU of the measured positions is compared to theentries in the second memory MEM2 by identification unit IU. Thedistance dxP1toP2@WU of the measured positions is equal to the firstmemory entry in second memory unit MEM2 containing dxP1toP2@W1.Therefore, the identification unit IU will identify the unidentifiedsubstrate as substrate W1.

If substrate W2 would have been fed into the lithographic apparatusinstead of substrate W1, dxP1toP2@WU would have been the distance of themeasured positions of substrate alignment marker P1@W2 and P2@W2. Thisdistance dxP1toP2@WU would have been equal to the second memory entry ofsecond memory unit MEM2 and the identification unit IU would haveidentified the unidentified substrate as substrate W2.

At least some embodiments of the invention can be used to correct fordifferences to the engineered status of the substrates. For example, thefirst circuit pattern CP1@W1 on substrate W1 is formed by projecting theimage of the first circuit pattern CP1@MA1 on patterning structure MA1onto substrate W1 via a first lithographic apparatus LA1. The firstcircuit pattern CP1@W2 on substrate W2 is formed by projecting the imageof the first circuit pattern CP1@MA1 on patterning structure MA1 ontosubstrate W2 via a second lithographic apparatus LA2. Due to an error insecond lithographic apparatus LA2, the relative position of the firstcircuit pattern CP1@W2 on substrate W2 and substrate reference markP2@W2 is not as defined in the associated memory. The error in secondlithographic apparatus LA2 is known from a previous measurement and thisinformation is shared with the first lithographic apparatus LA1.

It may be desired to image the second circuit pattern CP2@MA2 via thefirst lithographic apparatus LA1 onto a substrate WU without errorcorrection. The identity of the substrate WU is initially unknown to thefirst lithographic apparatus LA1. The identity of the substrate WU isdetermined as explained earlier. Based upon the identity of thesubstrate, the lithographic apparatus determines which correction mustbe done when imaging the second circuit pattern CP2@MA2 onto thesubstrate. If the substrate WU is identified as substrate W1, nocorrection is required. If the substrate WU is identified as substrateW2, a correction may be required. Second circuit pattern CP2@MA2 isimaged onto substrate W2 correcting for the error in lithographicapparatus LA2.

At least some embodiments of the invention can be provided with a deviceor other structure to take into account the position and/or measurementerrors associated with substrate alignment mark P1 and substratereference mark P2. The defined position of the x co-ordinate xP2@W ofthe substrate reference mark is equal for both substrates W1,W2. Thedefined position of the x co-ordinate xP1@W1 of the substrate alignmentmark on substrate W1 differs from the defined position of the xco-ordinate xP1@W2 of the substrate alignment mark on substrate W2.Apart from these defined differences, in practice errors will alsooccur. The relative distance dxP1toP2@Wl from substrate alignment markP1 to substrate reference mark P2 on substrate W1 as measured can beexpressed asrdxP1toP2@W1=dxP1toP2@W1+pe1+me1,where pe1 is a position error and me1 is a measurement error (see FIG.10). An example of a position error is the error made in the distancebetween the circuit pattern CP1 @MA1 on patterning structure MA1 and ofthe reference mark P2@MA1 during production of the patterning structureMA1. As explained the lithographic apparatus may image the circuitpattern CP1@MA1, reference mark P2@MA1 and/or alignment mark P1 @MA1separately. Where in practice the image of the circuit pattern CP1@MA1is applied to each target portion C (FIG. 1) on substrate W1 only oneimage of reference mark P2@MA1 and one image of alignment mark P1@MA1 isapplied to substrate W1. The imaging is done using information on therelative positions on patterning structure MA1 and the defined positionson substrate W1.

In the event the relative distance between reference mark P2@MA1 andcircuit pattern CP1@MA1 on the patterning structure MA1 is not measuredand accounted for during the steps of imaging the reference mark P2@MA1and the circuit pattern CP1@MA1 onto substrate W1, the relative distancerdxCP1toP2@W1 may not be equal to dxCP1toP2W1.

Examples of measurement errors are errors made by the mark sensor anderrors by the position sensor IF (FIG. 1).

For substrate W2 the corresponding distance when measured can beexpressed asrdxP1toP2@W2=dxP1toP2@W2+pe2+me2.

Both measured relative distances rdxP1toP2@Wl and rdxP1toP2@W2 maycontain error terms. Identification unit IU compares the measureddistance wD to the defined distances dxP1toP2@W1 and dxP1toP2@W2 betweensubstrate alignment mark P1 and substrate reference mark on substrate W1and substrate W2. It is possible that none of the defined distancesdxP1toP2@W1 and dxP1toP2@W2 will be equal to the measured distance wD.Identification unit IU will determine the difference between themeasured distance wD and each of the defined distances. The defineddistance with the smallest difference to the measured distance wD may beselected as identifying the substrate. In the event|wD-dxP1toP2@W1|<|wD-dxP1toP2@W2| the identification unit IU willidentify the substrate as substrate W1. In the event that|wD-dxP1toP2@W1|>|wD-dxP1toP2@W2| the identification unit IU willidentify the substrate as substrate W2.

On the second substrate W2 the defined position of the x co-ordinatexCP1@W1 for the first circuit pattern is equal to that on substrate W1.On the second substrate W2 the defined position of the x co-ordinatexP2@W2 is also equal to that on substrate W1. On both substrates W1,W2the position of the first circuit pattern CP1@W1, CP1@W2 can bedetermined by measuring the x co-ordinate xP2@W1, xP2@W2 of thesubstrate reference mark and accounting for the defined relativepositions of the first circuit pattern CP1@W1, CP1@W2 and the substratereference mark P2@W1, P2@W2. This distance will be referred to as dCP1toP2@W. The relation isxCP1@W=xP2@W+dCP1toP2@W.  (1)

Identification unit IU is arranged to be able to read the positionsxCP1@W and xP2@W from memory unit MEM1 and to compute this distancedCP1toP2@W.

On the processed substrate W1 the distance will be referred to asrdCP1toP2@W1. The relation with the defined distance dCP1toP2@W isrdCP1toP2@W1=dCP1toP2@W1+ε1.  (2)

The term ε1 is a position error similar to position error pe1.

The defined distance between first substrate pattern CP1@W1 andsubstrate alignment mark P1@W1 is referred to as dCP1toP1@W1. Thedistance as realised on substrate W1 will be referred to as rdCP1toP1@W1and can be expressed asrdCP1toP1@W1=dCP1toP1@W1+δ1,  (3)wherein δ is a position error similar to position error ε1.

Likewise, on substrate W2, defined distance between first circuitpattern CP1 @W2 on substrate W2 and substrate alignment mark P1@W2 willbe dCP1toP1@W2 and the realised distance will be rdCP1toP1@W2. Therelation can be expressed asrdCP1toP1@W2=dCP1toP1@W2+δ2,  (4)wherein δ2 is a position error similar to position errors δ1 and δ1.

The measured x co-ordinate xP2@Wl of substrate reference mark P2@W1 canbe expected to be atxP2@W1=xP2@W1+ξ1.  (5)

The term ξ1 is a measurement error similar to measurement error me1.

The measured x co-ordinate xP1@W1 of substrate alignment mark P1@W1 canbe expected atxP1@W1=xP1@W1+ζ1.  (6)

The term ζ1 here also is measurement errors. This error does not need tobe equal to the measurement error ξ1, for instance because of noise.

Once both the substrate reference mark P2@W1 and the substrate alignmentmark P1@W1 are formed on substrate W1 and substrate W1 is developed,both their positions can be read and can be used to determine theposition of the first circuit pattern CP1@W1 on substrate W1.

The position of the first circuit pattern on substrate W1 can beestimated fromxCP1@W1=xP2@W1+dCP1toP2@W1.  (7)Note that here the defined distance between CP1toP2@W1 is used insteadof the realised distance, since the realised position of first circuitpattern xCP1@W1 can not be measured.

The position of first circuit pattern on substrate W2 can be derivedfromxCP1@W2=xP2@W2+dCP1toP2@W2.  (8)

The position of the first circuit pattern CP1@W1 on substrate W1 canalso be estimated from a measured position of substrate alignment markP1@W1. This can be done viaxCP1@W1=xP1@W1+dCP1toP1@W1  (9)

For substrate W2 the position of the first circuit pattern CP1@W2 can beestimated viaxCP1@W2=xP1@W2+dCP1toP1@W2.  (10)

After identification of the substrate, it is known if the substratecontains substrate alignment mark P1@W1 or P1@W2, i.e. if the substrateis substrate W1 or substrate W2. In the event the substrate is substrateW1, the position of the first circuit pattern can be estimated usingeither the measured position of the substrate reference mark (formula 7)or the measured position of the substrate alignment mark (formula 9).The estimation can also use the measured position of both the substratereference mark and the measured position of the substrate alignment markin order to reduce the error terms. The effect of adding the twoestimations of formula 7 and 9 and dividing the result by 2 isxCP1@W1=(xP2@W1+dCP1toP2@W1+xP1@W1+dCP1toP1@W1)/2.  (11)

Filling in formula 5,2,6 and 3 clarifies how the errors translate intothe estimated XCP1@W1xCP1@W1=(xP2@W1+ξ1+rdCP1toP2@W1−ε1+xP1@W1+ζ1+rdCP1toP1@W1−δ1)/2.  (12)

In the event that more substrate alignment marks are used, the aboveexpression can be changed accordingly to minimise the estimation error.Using more substrate alignment marks of course also opens thepossibility to uniquely identify a larger set of substrates.

In a lithographic production process, alignment markers or features maybe used to code various properties associated with substrates. In oneembodiment of the invention, the properties may identify the substrateand may identify the type of process to be applied to the substrate.Additionally, the properties may include alignment informationassociated with the substrate. In another embodiment, the system mayadjust alignment values of the system for particular substrates based oninformation derived from the features on the substrates. A lithographicsystem may analyze the plurality of markers and identify thecorresponding substrates.

Conventional systems may serial numbers that are written on a substrateto identify the corresponding substrate. While the serial numbers mayenable identification of the substrates, their placement relative to thesubstrate edge is difficult to capture using a stepper capture range.

The invention enables identification of substrates based on informationthat is coded on a substrate using spatial coordinates for a pluralityof features. The features may be oriented relative to primary alignmentmarkers. For example, a plurality of alignment markers may generate acode using their position offset. The features may be positioned on afirst side or a second side of a substrate.

The invention provides features in x- or y-directions that may belocated with respect to one of the primary alignment markers. In oneembodiment of the invention, the features may occupy a space 30 mm longand 0.25 mm wide or any other dimensions. The features may include codeswith 50^49 digits, which the system may scan in ¾ seconds or less time.The code may be used as a barcode to identify substrate properties, suchas a type of substrate or may identify a particular substrate from abatch of substrates. The code may provide information that is used tocontrol operation of a stepper.

The invention may include feature arrangements having a primary markerthat is accurately placed relative to a known position on the substrate.Additional features may be positioned relative to the primary marker tocode alignment information, such as an amount the substrate should beshifted in the x-, y-, or z-directions in order to line up with aselected element. Imaging and readout tolerances may provide limitationsof code resolution for one marker arrangement. For example, if theposition resolution for imaging and readout is 500 nm and the maximumoffset range for the marker is 50 μm, a code resolution of 100² may beattained per marker and 10²⁴ for six markers.

The position of the substrate alignment mark P1 is drawn in FIG. 1 to bein an area similar to the target areas C. Because of the curvature ofthe substrate, on the edge of the substrate there are areas which aretoo small to fit a complete circuit. These areas are called mouse-bites.Mouse-bites can advantageously be used to contain substrate referencemarks or substrate alignment marks, thereby freeing target areas C forpatterning circuits.

The lines between the target areas C are commonly referred to inlithography as scribelanes. The circuits are separated from each otheralong these scribelanes. The scribelanes can advantageously be used tocontain substrate reference marks or substrate alignment marks, therebyfreeing target areas C for patterning circuits. Alternatively, thesubstrate reference marks or substrate alignment marks may be located ona substrate to be opposite the patterning circuits. Other configurationsmay be used.

In the embodiments above, the substrate alignment marks and substratereference marks are read after processing of the substrate, prior toimaging a subsequent layer. In specific circumstances it is possible toread the markers without further processing (e.g. subsequent toexposure, or subsequent to development of an exposed resist layer). Inthis case, the markers may be latent. It will be clear to a personskilled in the art, that latent markers can be used in embodiments ofthe invention.

It will be clear to a person skilled in the art, that any feature on thesubstrate or of the substrate of which the position can be determined,could replace the substrate reference mark. It will be clear to a personskilled in the art, that the relative positions of the substratereference marks and the substrate alignment marks may indicate orcontain information characterising the substrate such as a date, aserial number processing information, factory information, or otherinformation. It may also identify the number of substrates within aseries with the same characteristics. Together with the serial number,for instance 7, the number of substrates within a series of for instance9 substrates would indicate that it concerns substrate number 7 of 9substrates. In all these cases, this characterising informationregarding the substrate may be encoded in the relative positions. Thecharacterising information regarding the substrate can be decoded with aknown relation between the relative positions and the characterisinginformation corresponding to certain relative positions. It will beunderstood that the characterising information regarding the substratesuch as date, serial number, processing information, factoryinformation, or other information, can be considered to identify asubstrate or set of substrates.

It will be clear to a person skilled in the art, that the characterisinginformation may be used to calibrate the lithographic apparatus. Forinstance, the identity of a calibration substrate may be associated withheight information such as the difference in height between twopositions on the substrate (x1,y1,z1),(x2,y2,z2) (not shown).Differences in height are distances along the z-axis (FIG. 1). Themeasured distance is compared with a previously measured distanceaccording to the characterising information. The ratio between thepreviously measured distance and the measured distance can be used as acalibration ratio. Multiplying measured z-co-ordinates on thecalibration substrate with the calibration ratio will result incalibrated z-co-ordinates. In other words, the lithographic apparatus iscalibrated.

The calibration ratio may also be used to calibrate measurements onother substrates W. The z-co-ordinates measured on other substrate W aremultiplied with the calibration ratio to give a calibratedz-co-ordinate.

It will be clear to a person skilled in the art, that within each layeron the substrate new characterising information regarding the substratecan be imaged. This can be realised by imaging a new set of an alignmentmarker and a reference marker onto the substrate. The new set of markerscan for instance be imaged into the scribelanes of the substrate.

In the embodiments of the invention described above an off-axis marksensor MS (FIG. 1) is used. The mark sensor MS could equally well havebeen on-axis. If the measurement by the sensor is performed by holdingthe mark so that it crosses the optical axis of the projection systemPL, the sensor is called an on-axis sensor.

Specifically in a system with two substrate tables (not shown), themeasurement with the mark sensor MS of substrate W2 can be performedsimultaneously with illumination of substrate W1 using the projectionsystem PL. This way the identification can be completed before thesubstrate is brought underneath the projection system PL.

In the embodiments above, the patterning structure MA1 may contain analignment mark M1 and a reference mark M2. Because the marks are imagedseparately onto substrate W1, it will be clear to a person skilled inthe art that only alignment mark M1@MA1 is required on patterningstructure MA1. The provision is that the alignment mark M1@MA1 onpatterning structure MA1 is imaged onto substrate W1 at the position ofsubstrate alignment mark P1@W1 and at the position of substratereference mark P2@W1. Here the relative positions of substrate alignmentmark P1@W1 and P2@W1 can be defined to characterise informationregarding substrate W1.

An identification unit, as described herein, may include one or morearrays of logic elements, such as microcontrollers, microprocessors, orother processing units. Such an array may be configured to executesoftware and/or firmware instructions. Alternatively, such an array mayat least in part be hard-wired (e.g. an application-specific integratedcircuit). As a further alternative, such an array may be fixed butreprogrammable (e.g. a field-programmable gate array).

FIG. 11 shows a cross section of the lithographic apparatus of FIG. 1 ata different y-position compared to FIG. 1. In an embodiment of theinvention, a source S of measurement radiation is fixed on a frame F(or, has a known position relative to frame F). The measurementradiation is directed to an alignment mark M1 on mask MA. Sensordetection optics DO form an image of the alignment mark M1 onto a cameraCAM (having a CCD, CMOS, or other such sensor). A representation of theimage formed onto the camera CAM is retrieved by a position processingdevice PPD. Position processing device PPD (e.g. a processor, embeddedprocessor, or other array of logic elements executing a set ofinstructions in firmware and/or software) determines the position of themask. The position processing device PPD may receives input fromposition sensor IF2, camera CAM and input device IP. Position sensor IF2may be fixed to frame F (or, has a known position relative to frame F)and measures the position of the first positioner PM. In thisembodiment, position sensor IF2 is an interferometer which has its basefixed to frame F and its moving part on the first positioner PM. Thus,the location of the first positioner PM is known relative to the framewhen camera CAM is read. Input device IP is a keyboard, touch screen,mouse, or other device for data entry.

FIG. 12 schematically shows the region of the mask which contains thealignment mark. Also shown in FIG. 12 is the xy coordinate system ofFIG. 1. This region on the mask is referred to herein as the alignmentregion. The alignment region can be much larger than the area which ismeasured during alignment, the measurement area. Valid alignmentmeasurements can be performed at multiple positions of the measurementarea within the alignment region.

The alignment region M1 may include a number of features (indicated inFIG. 12 by dots, one dot indicating one feature) distributed over thealignment region M1. The features are formed by small areas with highreflectivity which reflect radiation emitted by the measurementradiation source S. The area surrounding the features is the backgroundand has lower reflectivity.

Reference information indicating the positions of the features is storedin position processing device PPD, relative to an origin ORm of aCartesian coordinate system of the alignment region M1. The positions ofthe features are designed so that the distances between the features inx- and y-directions can identify the features itself. For instance, onlyfeature F1 has a distance to the next neighbor in the positivex-direction of 1 unit and has a distance to the next neighbor in thepositive y-direction of 1 unit. Only feature F2 has a distance to thenext neighbor in the positive x-direction of 5 units and has a distanceto the next neighbor in the positive y-direction of 3 units.Additionally, by determining the position of any one feature, theposition of any other feature may be calculated.

For a given feature, position processing device PPD determines thedistances in the positive x- and y-direction to the next features, usingthe stored positions of the features. Position processing device PPDstores the distances in x- and y-directions for that feature in a table.This may be repeated for the other features.

Only a part of the total alignment region M1 is imaged by detectionoptics DO. This is the measurement area IA1. The measurement area IA1,indicated by the dashed line in FIG. 12, is visible to the camera CAM.The measurement area IA1 contains four features, one being F1, whichform a subset of the features in alignment region M1.

The output of the camera CAM comprises an image of the measurement areaIA1 (shown in FIG. 13) converted to electronic data. Position processingdevice PPD may have no prior knowledge on the identities of the fourfeatures in the measurement area IA1 indicated by dots and may label thefeatures as U1, U2, U3 and U4. Position processing device PPD determinesthe positions of features U1, U2, U3 and U4 relative to an origin ORs ofa Cartesian coordinate system of the camera CAM (e.g. of the camera'ssensor). One unit in the Cartesian coordinate system of the camera CAMcorresponds to one unit in the Cartesian coordinate system of thealignment region. The positions are used to calculate and store thedistances of the features in the measurement area IA1 to the nextfeatures in the positive x- and y-direction. The distances are storedfor each of the unidentified features (i.e. U1, U2, U3 and U4).

Feature U1 in image IA1 has a distance to next neighbor in the positivex-direction of 1 unit and also has a distance to the next neighbor inthe positive y-direction of 1 unit.

Since only feature F1 in alignment region M1 has a distance to the nextneighbor in the positive x-direction of 1 unit and a distance to thenext neighbor in the positive y-direction of 1 unit as well, positionprocessing device PPD identifies feature U1 in image IA1 as feature F1in alignment region M1 using the calculated distances.

Using the identity of feature U1, the position of the alignment regioncan be determined relative to the position of the camera CCD. Thepositions of the features are stored in position processing device PPDrelative to the origin ORm of the alignment region. In this example thecoordinates of feature F1 in the alignment region are (2,8), where thefirst numbers between brackets indicate the x-coordinate and the secondnumbers indicate the y-coordinate. The coordinates of the identifiedfeature U1 in the image IA1 are known relative to the origin ORs of thecamera CAM and are (1,1). The position of the origin ORm of thealignment region with respect to the origin ORs of the camera CAM ORs isdetermined by:ORm=CoordinatesU1−CoordinatesF1,where CoordinatesU1 stands for the coordinates of feature U1 and whereinCoordinatesF1 stands for the coordinates of feature F1.

In the example of FIG. 12 and FIG. 13 and using F1, the position of theorigin ORm of the alignment region M1 in the coordinate system of thecamera CAM is ORm=(1,1)−(2,8)=(−1,−7).

The position of the alignment region M1 on the mask MA is known.Therefore, the mask can be aligned with respect to the lithographicapparatus.

It will be appreciated by a person skilled in the art, that it is notnecessary to store the positions of all features in alignment region M1in position processing device PPD. The features for which no positionsout of the reference information are stored are either not measured, ortheir measured positions do not lead to an identification.

FIG. 14 shows an alignment region M2 in an embodiment of the invention.The dots in FIG. 14 indicate individual features. Only part of the totalalignment region M2 is imaged by detection optics DO. This is themeasurement area IA2. The measurement area IA2 is indicated by a dashedline and contains two features F5, F6.

The output of the camera CAM comprises an image of the measurement areaIA2 (shown in FIG. 15) converted to electronic data. Position processingdevice PPD may have no prior knowledge of the identities of the featuresin measurement area IA2. Position processing device PPD determines thepositions in the coordinate system of the camera CAM of the two featuresin the measurement area IA2 as (0.5,2.5) for U5 and (0.5,0.5) for U6.The positions are used to determine and store the distance of thefeature U6 in the measurement area IA2 to feature U5, the next featurein the positive y-direction. This distance is stored.

Feature U5 does not have a neighbor in the positive x-direction. Theedge of the image IA2 is at 2.5 units in the positive x-direction. Onlyfeatures F3, F4, F5 and F6 in the alignment region M2 do not haveneighbors in the positive x-direction within a distance of less than 2.5units. Therefore feature U5 is identified as anyone of features F3, F4,F5 or F6.

Feature U5 does not have a neighbor in the positive y-direction, whereasthe distance of feature U5 to the edge of image IA2 is 0.5 units. In thetable stored in the position processing device PPD, all distances toneighbors in the positive y-direction are at least 1 unit. Therefore itcannot be identified further.

Feature U6 has a neighbor in the positive y-direction at a distance of 2units. Since only features F7, F8 and F6 have the nearest neighbors inthe positive y-direction at a distance of 2 units, feature U6 isidentified as anyone of features F7, F8 and F6.

With feature U6 identified as being one of features F7, F8 or F6,feature U5 is identified as being one of features F9, F10 or F5. Sincefeature U5 was already identified as being one of features F3, F4, F5 orF6 the combination leads to the identification of feature U5 as featureF5.

The position of the origin of the alignment region in the coordinatesystem of the camera CAM can be found using the identity of feature F5asORm=CoordinatesU5−CoordinatesF5where the coordinates CoordinatesF5 of feature F5 are in the coordinatesystem of the alignment region, and the coordinates CoordinatesU5 offeature U5 are in the coordinate system of the camera CAM. This thenallows alignment of the mask with respect to the lithographic apparatus.

It will be appreciated by a person skilled in the art that it may bedesirable to design the alignment region such that the measurement areawill contain a sufficient number of features to uniquely identify atleast one feature in each possible image. For instance the features canbe placed in the alignment region randomly. It can be assumed that asufficient number of features is present in each possible image, bychoosing a high density of features.

There may be a solitary feature in the alignment region, that is remotefrom all other features in the alignment region. If the measurement areais placed over such a solitary feature and none of the other features ispresent in the measurement area, the solitary feature may still beidentified. Identification is performed by checking if there is no otherfeature in the reference information stored in position processingdevice PPD, that can be in the measured position relative to the cameraCAM without other features visible to the camera CAM.

In an embodiment of the invention, measured distances between featureson a mask can be used to identify the alignment region or even the mask.Mask MA3 includes an alignment region M3 (FIG. 16) and mask MA4comprises an alignment region M4 (FIG. 16). Alignment region M4 has thesame size as alignment region M3. Both alignment regions M3,M4 contain abasic feature F1 (indicated in FIG. 16 by a dot) at coordinates (2,10)and contain the same pattern of features (indicated in FIG. 16 by dots).However all distances to the basic feature are multiplied by a factor 2in alignment region M4 relative to the distances in alignment region M3.Some features which are present in the alignment region M3 do not have acounterpart in alignment region M4 because their distance to the basicfeature F 1 is so large that they fall outside alignment region M4. Thedistances in each of the axial directions in alignment region M3 arechosen to be odd numbers of units (such as 1,3, . . . ). This means thatthe distances in each of the axial directions in alignment region M4 areeven numbers of units (such as 2 and 6). Measuring the distance between2 features with the same x-coordinate will now identify the features andthe alignment region M3 on mask MA3 or alignment region M4 on mask MA4as well. Part of the alignment region M4 is imaged by detection opticsDO (FIG. 1). FIG. 17 shows the image IA3 visible to camera CAM.

FIG. 17 shows features U9 and U10 indicated by dots in image IA3 in FIG.16. The distance between features U9 and U10 is (2,0). Therefore, thealignment region from which the image is taken is identified as M4.Since alignment region M4 is present on mask MA4 and not on mask MA3,the mask is identified as mask MA4.

It will be appreciated that because the alignment region, or even themask on which the mark is present, can be identified, the alignmentregion could be described as information range (in the sense that itprovides identity information).

It will be appreciated that there are a number of alternatives tofeatures having a relatively high reflectivity and to area between thefeatures having a lower reflectivity:

1. The features may be formed by spots with low reflectivity forspecific radiation. The area outside the spots would have higherreflectivity for that radiation.

2. The features may be formed by spots with a different transmissionthan the background for specific radiation. The mask would then beplaced between a sensor (in case the sensor is a position sensitivedevice such as a CCD-camera) or set of sensors (in case the sensors arenot position sensitive devices, e.g. the sensors are photocells) and theradiation source.

3. The features may be formed by spots which scatter specific incomingradiation in a direction different from the direction into which thebackground scatters the incoming radiation

4. The features may be formed by spots which diffract specific incomingradiation in a direction different from the direction into which thebackground diffracts the specific incoming radiation.

5. The features may be areas which are elevated above the rest of thealignment region. By radiating light parallel to the measurementsurface, the presence of such a feature will block the radiation. Asensor opposite to the radiation source will now be able to detect thepresence of a feature.

Other types of features can be used for the invention, as will beappreciated by the person skilled in the art.

It will be appreciated that any device capable of determining theposition information of features in the alignment region can be used.The features can be circuit features such as pick and place components,vias or conducting areas.

It is not necessary that the sensor include one measurement device. Anyset of suitable detectors with known relative positions, can be used asa sensor to determine the position of features in the alignment region.Each measurement device detects the presence or absence of a feature inits measurement area. The positions of detected features are indicatedby the identity of the measurement devices which detected the features.Examples of such devices are air gauges (capable of determining localheights), capacitance sensors (capable of determining local capacitance)and photodiodes. It will be appreciated that different detectionprinciples correspond to the different measurement devices.

The detection optics DO may not be perfect and may cause aberrations inimaging the measurement area onto the camera CAM. For instance, theaberrations may be smallest in the center of the image. In such case,the accuracy of the measured position may be optimized by using only thepositions of features in the center of the image. After identifying afirst feature, a second feature close to the center may be selected andthe measured position of that second feature may be used for theposition determination of the alignment region.

When the sensor is first used (and/or periodically thereafter), it maybe desirable or necessary to calibrate it. The sensor is calibratedusing the measured positions and the positions stored in processingdevice PPD (FIG. 11). Having identified features F5 and F6 (FIG. 14),the measured distance can be compared with the distance determined fromthe positions stored in processing device PPD. The comparison isobtained by determining a ratio between the measured distance and thedistance determined by the positions stored in processing device PPD.The ratio links 1 unit distance in the coordinate system of the sensorto 1 unit distance in the coordinate system of the mask MA. The sensoris calibrated by dividing the measured distances to the origin of thesensor by the determined ratio.

It will be understood by a person skilled in the art that otherparameters which influence measurement by the sensor (such as rotation,field distortion and aberrations) can also be calibrated. For thiscalibration it may be desired to use the least squares criterion or anyother suitable criterion to identify measured features and at the sametime determine a parameter of the transformation going from thecoordinate system of the mask MA to the coordinate system of the sensor.The alignment region of the marker as measured by the sensor may in somecases be rotated or expanded. For instance it can have any rotationalangle around any axis, in addition to having an unknown position at themoment of measuring the positions of features in the measurement areawith a sensor. Further possible transformations are symmetric rotation,asymmetric rotation, symmetric magnification, asymmetric magnificationor higher order parameters such as position dependency of x^2 or xy.

This problem is solved in an embodiment of the invention by using aninverse transformation model with a parameter to be determined. Theinverse transformation is applied to the positions of the measuredfeatures, resulting in transformed locations as a function of theparameter. By applying the least squares criterion to the differencesbetween positions of features in the reference information, andtransformed positions of the measured features, one can find theparameter. In the most simple form of the model, the inversetransformation model is a translation. The outcome of the model is theposition. In another form of the model, the rotation angle around thex-axis is a parameter. This is determined simultaneously with theposition.

It will be understood by a person skilled in the art that atransformation model may equally well be applied to features in thereference table instead of to the measurements of the features. However,since there may be more features in the reference information than inthe subset of features that is measured, this may cause morecomputational effort than applying the inverse transformation model tothe positions of the measured features. It will also be understood by aperson skilled in the art that both the transformation model and theinverse transformation model may be applied to find several parameterssimultaneously.

Embodiments of the invention may be advantageously used when thedetection optics DO, shown schematically in FIG. 11, are telecentric onthe side facing the mask. Telecentricity is useful because it ensuresthat the distance between the features in the image does not depend onthe distance between the mask MA and the detection optics DO.Determining a position of the mask in x and y coordinates therefore isindependent of the z-coordinate of the mask.

A further benefit of using telecentric detection optics is that the sizeof the features U5,U6 (FIG. 15) as measured, can be used to determinethe distance of the mask MA from the detection optics DO. The distanceof the camera CAM to the detection optics DO is fixed at a knowndistance to the detection optics DO, defining an image plane. A sharpimage IA2 is formed when the alignment region on mask MA lies in anobject plane which is conjugate to the image plane of the detectionoptics. If the alignment region M2 is not in the related, conjugateobject plane, the features in the image IA2 may be blurred, andtherefore appear larger than when the alignment region M2 is in theconjugate object plane. The size of the features U5,U6 is measured andused to indicate the distance between the alignment region M2 and thedetection optics. With the known fixed distance of the camera CAM to thedetection optics DO, the size of the features U5,U6 indicates thedistance to the alignment region and the camera CAM.

A further advantage can be gained by using double telecentric detectionoptics. By using double telecentric detection optics, the distancebetween the features in the image does not depend on the distance of thecamera CAM to the detection optics. Therefore an accurate determinationof the distance between the camera CAM to the detection optics is notrequired.

When the mask contains two features, the position and orientation of themask can be determined, unless the mask has been rotated by 180 degreesaround a point midway between both features. When this occurs, thefeatures swap position, and the measurements may mistake one feature forthe other. Depending on the exact identification method, the swap in theidentification might also take place for a rotation of 90 degrees.

Because of the handling of masks in a lithographic productionenvironment the orientation will be known within much better accuracythan 90 degrees, and the problem is avoided. In the event an unknownrotation may have occurred, the mask may need to contain at least threefeatures A,B,C at unique positions. Unique positions means that thedistance between A and B differs from the distance between B and C andfrom the distance between A and C. The measurement of the position anddistance between any two features forming a subset of features can becompared to the reference information stored in position processingdevice PPD. One of the three reference distances will give the closestmatch to the measured distance. In the event the positions of thefeatures A and B have been determined, then to determine theorientation, the feature C, which can be considered to form a furthersubset of features, is also measured. The further subset of featurescould contain additional features, which may also be used to measure therotation.

Independent measurements may be used to improve the accuracy of a methodaccording to an embodiment of the invention. For instance the positionof the mask may be determined using a first and a second (e.g. CCD,CMOS) camera, each measuring a respective one of two alignment regionswith known relative positions. The relative positions of the first andthe second cameras is known. The measured position for each of the twoalignment regions indicates the position of the mask. The measuredpositions of the alignment regions are compared with their expectedpositions. This measures the rotation of the mask relative to the firstand second cameras. To reduce the measurement noise, the positionsdetermined with the first and the second cameras may be averaged.Alternatively the measurements may be repeated. Where this is done, themeasurements for which the position information of the first cameramatches best with the position information of the second camera, isselected as being the correct measurement. In addition, severalmeasurements with one camera CAM of one alignment region can beaveraged.

The position of the mask, determined using a method according to anembodiment of the invention, may be used to control the relativepositions of the mask and the substrate such that a projection beam ofradiation patterned by the mask will be exactly (e.g. to a high degreeof accuracy, such as nanometer) on top of a target area on thesubstrate. Alternatively, the position of the mask may be controlled,with the substrate maintained in a fixed position. Alternatively, acontrol unit may control the position of the substrate based upon theposition of the mask, with the mask maintained in the determinedposition. A control unit may be provided to control the relativeposition of the mask and the substrate.

In an advantageous use of an embodiment of the invention, the rotationof the mask relative to the sensor is determined and then adjusted to adesired angle. The desired angle is such that the image of the maskformed by the projection system PL has the same rotation as the targetarea on the substrate. Alternatively the rotation of the substrate maybe adjusted.

It will be appreciated by the person skilled in the art, that thereference information may be provided in different ways. The referenceinformation can be stored in a separate memory unit (e.g. semiconductormemory or one or more disk drives) connected to the position processingdevice PPD.

For the purpose of explaining principles of the invention, examples aregiven of an alignment region on a mask. It will be appreciated thatembodiments of the invention may be practiced on substrates as well, oron an object provided with an alignment region having suitable features,the object being supported by an object table, for example a substratetable or mask table. Alternatively, the object table may be providedwith an alignment region having suitable features.

According to one embodiment, a method of determining a position of anobject provided with a plurality of features having unique relativepositions, in a lithographic apparatus having a sensor, includesproviding reference information relating the positions of the featureswherein one position is an absolute position; characterized by measuringthe position of a subset of the features using the sensor wherein atleast one position is an absolute position in a coordinate system; andidentifying one or more of the measured features based upon the uniquerelative position of the one or more features with respect to the otherfeatures; determining the position of the object using the identity ofthe one or more identified features, the reference information and oneor more of the measured absolute positions in the coordinate system.

A lithographic apparatus according to a further embodiment includes anobject table for supporting an object, the object being provided with aplurality of features having unique relative positions; a positionsensor arranged to detect a subset of the features on the object;characterized by; a memory unit to store reference information relatingthe positions of the plurality of features, wherein one position is anabsolute position in a coordinate system; a processing device connectedto the position sensor and to the memory unit, arranged to identify oneor more features out of the measured subset of features, based upon theunique relative position of the one or more features with respect to theother features and arranged to determine the position of the objectrelative to the sensor using the reference information and the measuredabsolute position in the coordinate system.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described. The description is not intended to limit theinvention. For example, embodiments of the invention also includecomputer programs including one or more sets (e.g. sequences) ofmachine-executable instructions describing one or more methods asdisclosed herein, and data storage media (e.g. semiconductor memory(volatile and/or non-volatile), magnetic and/or optical disk media,etc.) containing such instructions. The scope of the invention is to bedetermined solely by the appended claims.

1. A lithographic apparatus comprising: a sensor arranged to measurepositions of first and second features on a substrate; an identificationunit, operatively connected with at least one memory and arranged tocompare a relative position of the first and second features, saidrelative position being based on the measured positions, with at leastone of a plurality of relative positions of corresponding first andsecond features, stored in said memory, each of the plurality of storedrelative positions of first and second features being associated withinformation characterising at least one substrate to distinguish said atleast one substrate from other substrates that have different relativepositions of the corresponding first and second features, wherein theidentification unit is arranged to indicate a correspondence between therelative position of the first and second features and one of theplurality of stored relative positions of first and second features. 2.The lithographic apparatus according to claim 1, wherein the informationcharacterising at least one substrate indicates at least one of: asubstrate identity, height information of a calibration substrate, aquantity of substrates in a set of substrates to which the substratebelongs, a date of an earlier process operation undergone by thesubstrate, a time of an earlier process operation undergone by thesubstrate, an apparatus used in an earlier operation of the lithographicprocess, and a patterning structure used in an earlier operation of thelithographic process.
 3. The lithographic apparatus according to claim2, wherein the identification unit is arranged to indicate acorrespondence between the relative position of the first and secondfeatures and a stored relative position of first and second features,which is closest in value to the relative position.
 4. The lithographicapparatus of claim 1, wherein the relative position of the first andsecond features includes a first distance in a first direction and asecond distance in a second direction different from the firstdirection, and wherein the identification unit is arranged to comparethe first distance with at least one stored distance in the firstdirection, the at least one stored distance in the first direction beingindicative of information about a corresponding substrate, and whereinthe identification unit is arranged to compare the second distance withat least one stored distance in the second direction, the at least onestored distance in the second direction being indicative of informationabout the corresponding substrate.
 5. The lithographic apparatus ofclaim 1, wherein the apparatus includes a positioning structure arrangedto position the substrate based on a position of the first feature asmeasured by the at least one sensor.
 6. The lithographic apparatus ofclaim 1, wherein the apparatus includes a positioning structureconfigured to determine a position of the substrate based on the one ofthe plurality of stored relative positions of first and second features.7. The lithographic apparatus of claim 1, wherein the apparatus includesa calibration structure configured to calibrate the apparatus based onthe information characterising at least one substrate associated withthe one of the plurality of stored relative positions of the first andsecond features.
 8. The lithographic apparatus of claim 1, wherein theidentification unit includes an array of logic elements and a memorystoring instructions executable by the array of logic elements.
 9. Thelithographic apparatus of claim 1, wherein the substrate includes aplurality of features having unique positions relative to one anotherand wherein the sensor is configured to measure positions of theplurality of features relative to a reference position, the apparatusfurther comprising: a memory unit that is configured to store referenceinformation indicating, relative to the position of the referenceposition, the location of the plurality of the features; and a processordevice, coupled to the sensor and to the memory unit, that is configuredto identify the first and second features from among the plurality offeatures.
 10. The lithographic apparatus of claim 1, wherein thesubstrate includes a target portion positioned on a first side of thesubstrate and a feature portion positioned on a second side of thesubstrate, the second side being located opposite to the first side, thesecond side having a plurality of features located thereon, thelithographic apparatus further comprising: an optical system that isconfigured to transmit an image of the plurality of features from thesecond side of the substrate onto a plane that is located outside of aperimeter of the substrate.
 11. The lithographic apparatus of claim 10,wherein the optical system places the image of the plurality of featuresfrom the second side within a capture range of the sensor.
 12. A methodof obtaining information regarding a substrate, said method comprising:measuring positions of first and second features on a substrate andstoring the measured positions in at least one memory; comparing arelative position between the first and second features on thesubstrate, said relative position being based on the measured positions,with at least one of a plurality of relative positions of correspondingfirst and second features, stored in said memory, each of the pluralityof stored relative positions of the corresponding first and secondfeatures being associated with information characterizing at least onesubstrate to distinguish said at least one substrate from othersubstrates that have different relative positions of the first andsecond features; and indicating a correspondence between the relativeposition of the first and second features and one of the plurality ofstored relative positions of the corresponding first and secondfeatures.
 13. The method according to claim 12, wherein indicating thecorrespondence includes indicating the correspondence between therelative position of the first and second features and a stored relativeposition of first and second features, which is closest in value to therelative position.
 14. The method according to claim 12, wherein thesubstrate includes a plurality of features having unique positionsrelative to one another, the method further comprising: measuringpositions of the plurality of features, based on a reference position;and identifying the first and second features among the plurality offeatures.
 15. The method according to claim 12, further comprising:projecting an optical beam onto a feature portion of a second side ofthe substrate that includes a plurality of features, the second side ofthe substrate being located opposite to a first side of the substratehaving a target portion; and creating an image of the plurality offeatures from the second side of the substrate on a plane that islocated outside a perimeter of the substrate.
 16. The method accordingto claim 15, further comprising placing the image of the plurality offeatures within a capture range to enable measuring of the positions ofthe first and second features on the substrate.
 17. A devicemanufacturing method comprising: manufacturing a plurality of devices ona set of substrates, each substrate having a marker and a feature at aunique position relative to the marker; for at least one of the set ofsubstrates, measuring a position of at least one of the feature and themarker relative to the other of the feature and the marker, and storingthe measured position in at least one memory; comparing the measuredrelative position to at least one of a plurality of entries in a dataset stored in the memory, each entry corresponding to a relativeposition between the marker and feature of one of the set of substrates;and identifying the substrate by selecting an entry corresponding to themeasured relative position to distinguish the substrate from othersubstrates that have different relative positions of the marker and thefeature, wherein said manufacturing includes modifying a processoperation based on the identity of the substrate.
 18. A method oflabeling a substrate, said method comprising: providing the substratewith a first feature; providing the substrate with a second feature; andrecording and storing in at least one memory a correspondence between arelative position of the first and second features and informationcharacterizing the substrate to distinguish the substrate from othersubstrates that have different relative positions of corresponding firstand second features.
 19. The method according to claim 18, wherein thesubstrate is part of a set of substrates; and wherein the informationcharacterizing the substrate is common to each substrate of the set ofsubstrates.
 20. The method according to claim 18, wherein the substrateis part of a set of substrates; and wherein the informationcharacterizing the substrate distinguishes the substrate from others inthe set of substrates.
 21. The method according to claim 18, wherein thesubstrate is part of a set of substrates; and wherein the informationindicates at least one of: the substrate identity, height information ofa calibration substrate, a quantity of substrates in a set of substratesto which the substrate belongs, the date of an earlier process operationundergone by the substrate, the time of an earlier process operationundergone by the substrate, an apparatus used in an earlier operation ofthe lithographic process, and a patterning structure used in an earlieroperation of the lithographic process.
 22. A device manufacturing methodcomprising: manufacturing a plurality of devices on a set of substrates,each substrate having a marker that indicates a position of thesubstrate and a feature at a position relative to the marker, andstoring the positions in at least one memory; and for one of the set ofsubstrates, determining a relative position of the marker and feature todistinguish said one of the set of substrates from other substrates thathave different relative positions of the marker and the feature, whereinsaid manufacturing includes selecting an aspect of a process operationon the substrate based on the determined relative position.
 23. Alithographic substrate comprising: a first feature; and a second featureat a relative position to the first feature, wherein the relativeposition indicates information that has been encoded onto thelithographic substrate, and stored in at least one memory, todistinguish the lithographic substrate from other substrates that havedifferent relative positions of corresponding first and second features.24. The lithographic substrate according to claim 23, wherein theinformation indicates at least one of: a substrate identity, heightinformation of a calibration substrate, a quantity of substrates in aset of substrates to which the substrate belongs, a date of an earlierprocess operation undergone by the substrate, a time of an earlierprocess operation undergone by the substrate, an apparatus used in anearlier operation of the lithographic process, and a patterningstructure used in an earlier operation of the lithographic process. 25.The lithographic substrate according to claim 23, wherein the firstfeature is created on the substrate at a first time, and wherein thesecond feature is created on the substrate at a second time separatefrom the first time.
 26. The lithographic substrate according to claim23, wherein the first feature is imaged onto the substrate by anexposure at a first time, and wherein the second feature is imaged ontothe substrate by an exposure at a second time separated from the firsttime by a period of non-exposure.